The LISA Pathfinder mission of ESA (formerly the mission was called SMART-2) is a technology demonstration mission for LISA, a kind of physics research laboratory in space, with the objective to test and verify the key technologies needed for highly accurate formation flying and precise measurement of the separation (metrology) between two very distant spacecraft.1)2)3)4)5)6)

Legend to Figure 1: The top left image shows the classical Einstein thought experiment to measure the spacetime curvature. This is the basis for all gravitational wave detectors, e.g. LISA (top right). LISA Pathfinder will not only pave the way for LISA, but will also demonstrate the main assumption of the thought experiment: that free particles follow geodesics.

Some background on the LISA (Laser Interferometer Space Antenna) mission:

LISA Pathfinder is a necessary precursor mission for LISA, the joint ESA and NASA formation-flying mission (launch planned for 2034) of three spacecraft. The goal of the LISA fundamental physics mission is to detect low-frequencygravitational waves in the range of 10-4 to 10-1 Hz (according to Einstein's Theory of General Relativity the force of large gravity changes, generated by a collapsing star or an entire galaxy, will produce tiny ripples of gravity waves) - requiring technologies that have so far never been tested. With the detection of these waves, which for example originate from black holes in close orbit around each other, an entirely new window to the universe will be opened.

In this concept, the three LISA spacecraft constellation form an equilateral triangle (a giant interferometer) with an armlength of 5 million km, inclined by 60º against the ecliptic (Figure 2). Plans call for LISA's trio of spacecraft to orbit the sun at the same distance as Earth, but trailing about 50 million km in orbit behind our planet (representing an angle of about 20º with respect to the Earth-Sun direction). 7)8)

The concept of detecting a gravitational wave with an interferometric configuration like LISA is being realized using a transverse and traceless (TT) coordinate system. The goal is to detect the "ripples of gravitational waves" by the measurement of the time variation that can be detected by the giant interferometer.

In the LISA measurement concept, each spacecraft houses two proof masses; changes in the distance between the proof masses are measured interferometrically to a level of 10 pm. To insure that the proof masses can follow as close as possible a purely gravitational orbit, their position inside the spacecraft is constantly being monitored. Capacitive and interferometric sensors determine the position of the spacecraft with respect to the proof masses. A drag-free system and proper shielding must counteract the disturbance forces of the orbiting spacecraft. 9)10)11)

Due to the required level of residual forces on the proof masses, LISA faces a number of technological challenges, such as tight requirements on magnetic cleanliness, thermal stabilization, charge management, and above all the internal gravitational balancing to minimize the gravity gradient forces on the proof masses. The most critical technologies, such as the drag-free system (or the disturbance reduction system) and the interferometry are planned to be addressed by the precursor mission LISA Pathfinder - with the ESA payload LTP and the NASA payload DRS.

Figure 4: Artist's view of LISA's yearly orbit around the Sun. The rotation of the triangle that the spacecraft form can be seen in the picture (image credit: ESA)

Relationship between LISA Pathfinder and LISA

From the outset, the LISA Pathfinder mission has been designed such that the technology can be directly transferred to LISA with little, or no, changes. However, as LPF is a pathfinder, and in order to keep costs down, the Level One mission performance requirement is relaxed with respect to the LISA hardware requirements. Specifically, the requirement on the relative acceleration noise of the test masses is relaxed by an order of magnitude in performance, and by a factor of thirty in frequency (Figure 5). 12)13)

• The graph on the left side is showing the Level One acceleration noise requirement of LISA Pathfinder and LISA. The line labelled LISA Pathfinder Requirement shows the value as listed in the Science Requirements Document. The line labelled LPF CBE shows the Current Best Estimate of the expected performance of the mission. The gap between the LPF CBE line and the LISA Requirement represents the extrapolation required to transfer the LPF technology to LISA.

• The graph on the right side is showing the Level One Displacement Noise requirements. As can be seen, the LISA Pathfinder requirement on the performance of the readout interferometer is approximately equal to the performance of the LISA local (test mass readout) interferometer. The line labelled LPF CBE is the measured performance of the EM interferometer (optical bench, laser, phase meter).

The relaxation in the acceleration noise requirement significantly reduces the environmental requirements levied on the spacecraft. In particular, this relaxation is most evident on the thermal stability, gravitational balancing and magnetic cleanliness of the spacecraft. The environmental relaxation allows the LPF spacecraft to be in orbit around L1, as opposed to LISA's heliocentric Earth-trailing orbit. This has two advantages, namely in the time required to reach the desired orbit, and more importantly, it greatly reduces the communication requirements onboard the spacecraft (distance to LPF is approximately 1:5 million km as opposed to 50 million km for LISA).

Subsystem

LISA Pathfinder

LISA

Comments

LPF status

Laser

35 mW @ 1064 nm

2 W @ 1064 nm

LISA will use LPF-like laser as its master oscillator. Higher power is achieved by optical fiber amplifier

FM in final testing

Laser frequency stabilization

Unequal arm Mach-Zehnder

Unequal arm Mach-Zehnder or reference cavity

The laser onboard LISA requires several stages of frequency correction. The first stage, pre-stabilization, could adopt an LPF type unequal arm-length Mach-Zehnder interferometer as the frequency discriminator

Fully tested using EMs of optical bench, laser and phasemeter

Modulator

AOM (Acousto-Optic Modulator)

EOM (Electro-Optic Modulator)

No demonstration of LISA electro-optic modulator on LPF

FM (Flight Model) in final testing

Optical bench

Hydroxy catalysis bonded Zerodur bench

Hydroxy catalysis bonded Zerodur bench

Demonstration of the LISA optical bench manufacturability and pathlength was one of the main technology developments in LPF. Several technologies have been developed including hydroxy-catalysis bonded ultra-stable optical bench, and quasi-monolithic fiber injector assemblies

FM under construction

Phasemeter

SBDFT (Single Bin Discrete Fourier Transform) algorithm

Digital PLL (Phase-Locked Loop)

LISA requires a high-frequency phasemeter due to the large Doppler shifts between the S/C. As LPF uses a significantly lower heterodyne frequency, it does not require such a sophisticated phasemeter

FM under construction

Inertial Sensor (IS)

The inertial sensor is the main component of LISA which cannot be tested on the ground. The demonstration of the inertial sensor performance is the main reason for flying LPF. The LPF inertial sensor has been designed as the LISA inertial sensor from the beginning

IS-Test Mass

46 mm Au:Pt cube

46 mm Au:Pt cube

The LPF test mass is identical to the LISA test mass

FMs delivered

IS-Electrode Housing

Molybdenum housing with gold-coated sapphire electrodes

Molybdenum housing with gold-coated sapphire electrodes

LISA electrode housing will be identical to the LPF electrode housing

FM replica delivered for testing. Flight units under construction

IS-Caging Mechanism

3 actuator design

3 actuator design

LISA caging mechanism will be identical to the LPF caging mechanism. It consists of the launch lock CMSS (Caging Mechanism Support Structure) and GPRM (Grabbing, Positioning and Release Mechanism).

GPRM FMs delivered. CMSS FMs under construction

IS-Vacuum enclosure

Titanium enclosure with getter pump

Titanium enclosure with getter pump and gate valve

The basic vacuum enclosure of LISA will be identical to the LPF vacuum enclosure with the exception of a gate valve which can be used to vent the interior to space

FMs under construction

Front End Electronics (FEE)

Differential Capacitive Bridge

Differential Capacitive Bridge

Due to frequency relaxation, LPF FEE performance has not been demonstrated at LISA's lowest frequency band. In some cases, LPF FEE is more difficult than LISA due to in-band actuation along the sensitive axes.

FMs in final testing

Charge management

Photoelectric discharge

Photoelectric discharge

Only change could be utilization of solid state UV light source as opposed to gas discharge lamp.

FMs delivered

Micro Newton Thrusters

FEEPs (Field Emission Electric Propulsion)/Colloids

FEEPs/Colloids

The demonstration of µN thrusters is one of the primary goals of the LPF mission. Both FEEP and colloid thrusters will be demonstrated. The results of LPF will determine which thruster will be chosen for LISA. LPF FEEP thrusters have been designed to meet the LISA lifetime requirements, although full LISA lifetime will not be demonstrated with LPF

Table 1: Relationship between the LISA technology and LISA Pathfinder, LPF status as of April 2009 (Ref. 12)

eLISA (evolved Laser Interferometer Space Antenna)

eLISA was born out the original joint NASA/ESA LISA mission, after NASA pulled out of the project. On April 8, 2011, NASA announced that it would be unable to continue its LISA partnership with the European Space Agency due to funding limitations. 14)

eLISA will be the first observatory in space to explore the Gravitational Universe. It will gather revolutionary information about the dark universe. The eLISA Consortium is convinced that a spaceborne low-frequency gravitational wave observatory to be launched in 2034 is the ideal tool to make progress in our understanding of the Universe. The eLISA Consortium submitted a White Paper to ESA describing the science case in May 2013.15)16)

In the aftermath, ESA started studying a variant of the original LISA concept, called eLISA in ESA's "Cosmic Vision Program 2015-2025". The eLISA mission will consist of a configuration (cluster) of three satellites, one mother and two daughter spacecraft, placed at the corners of an equilateral triangle with a side length of approximately two million kilometers, which will follow the Earth on its orbit around the sun at a distance of about 50 million km (i.e. the center of gravity of the cluster will follow the Earth at a phase angle of 20 º when viewed from the sun). Moreover, the entire configuration is inclined by 60º with respect to the orbital plane of the Earth around the Sun (i.e. the ecliptic plane). The mother spacecraft carries two, and each of the daughter spacecraft carry one free-flying test masses that will be kept as far as possible free of external disturbances. The mutual distances of the test masses from satellite to satellite will be measured by means of high-precision heterodyne laser-interferometry. 17)

In this way, the extremely small distance variations between the test masses of two satellites can be detected which are caused by the passages of gravitational waves. The required measurement accuracy of the distances amounts to typically 1/100 of the diameter of a hydrogen atom (10-12 m) at a distance of two million kilometers (for a broadband measurement in the frequency range from 1 to10 mH (milliHertz). The tiny orbital and attitude corrections which are necessary to keep each satellite centered on the test masses will be determined by a DFACS (Drag-Free Attitude Control System) using the measurements of inertial sensors. The attitude measurements will be converted into correctional motions via µN thrusters. Cold gas and colloid thrusters will be tested during the LISA Pathfinder mission.

Figure 6: The eLISA triangle's orientation changes during each orbit. This fact enables researchers to determine the direction of gravitational waves reaching eLISA.

LISA Pathfinder Mission

LISA Pathfinder has been introduced to mitigate the risks of the LISA mission. The main goal of the LISA Pathfinder mission is to demonstrate the concept of the gravitational wave detection using a single spacecraft: it will put two test masses in a near-perfect gravitational free-fall and control and measure their motion with unprecedented accuracy. This is achieved through state-of-the-art technology comprising the inertial sensor system, the laser metrology system, the drag-free control system and an ultra-precise micro-propulsion system.

• LISA Pathfinder's experiment concept is to prove geodesic motion by tracking two test masses nominally in free fall through laser interferometry with picometer (10-12 m) distance resolution. LISA Pathfinder will show that the relative parasitic acceleration between the masses, at frequencies around 1 mHz, is at least two orders of magnitude smaller than the value demonstrated so far or to be demonstrated by any planned mission.

• To demonstrate drag-free and attitude control in a single spacecraft with two proof masses

• Test the feasibility of laser interferometry at the level of accuracy envisaged for LISA

• Test endurance of the different instruments and hardware in the space environment.

The basic elements to achieve and prove geodesic motion are the following:

Table 2: Comparison of main features of missions requiring geodesic motion

Some background on the history of the various LPF mission development stages:21)

• The LPF (LISA Pathfinder) mission was initially proposed in 1998 under the name ELITE (European LISA Technology Experiment). A homodyne interferometer was to be flown. The planned launch date of ELITE was 2002.

• The ELITE proposal was further refined over the next two years, and in 2000 was submitted to ESA as SMART-2 (Small Missions for Advanced Research in Technology-2). The initial design of the SMART-2 mission comprised two formation-flying spacecraft to serve a LISA Pathfinder and also a Darwin Pathfinder payload objective.
However, after further studies, it was decided to focus the mission on a single spacecraft demonstrating drag-free control dedicated to the future LISA mission. The planned launch date of SMART-2 was 2006. 22)

• The renaming of the SMART-2 mission to "LISA Pathfinder" occurred in 2004 to account for the changed mission objectives (technology demonstrations for the LISA mission only). In October 2004, the ESA Science Program Council (SPC) approved the LTP Multi-Lateral Agreement, detailing the national agency responsibilities for the construction of the LTP.

• Originally, LISA Pathfinder/SMART-2 consisted of two payloads, namely LTP (LISA Technology Package) provided by ESA member states, and the NASA-provided DRS (Disturbance Reduction System), also known as ST7 (Space Technology 7). Each payload consisted of two gravitational reference sensors (GRS), an interferometric readout system, drag-free and attitude control system (DFACS), and micro-Newton thrusters.In October 2005, the NASA-provided DRS was descoped; the DRS now consists of micro-Newton thrusters and DFACS, and will rely on the LTP as its gravitational reference sensor.

• In February 2006, after the successful completion of the Mission Preliminary Design Review, LISA Pathfinder entered the Development Phase.

• Dec. 12, 2013: The Engineering Qualification Model of the Inertial Sensor Head (ISH) for LISA Pathfinder has passed a significant milestone. The integration of all the components of the ISH with perfect alignment, and the successful completion of qualification tests mark the first time that a heavy test mass inertial sensor has been assembled and successfully tested (Figure 40). 23)

• Fall 2014: The Science Module was retrofitted with three new side-panels onto which the cold-gas micro-propulsion equipment had been integrated. Functional verification of the spacecraft is progressing as planned, with the completed version of the flight software and using the FM microthrusters driving electronics. 24)

• Feb. 27, 2015: LISA Pathfinder's propulsion and science modules are leaving the UK for the last time. Airbus Defence and Space, the world's second largest space company, will ship the two modules to IABG (Industrie Anlagen Betriebs Gesellschaft), near Munich in Germany, for final system level testing. The spacecraft is scheduled to be launched later this year by a European Vega rocket from Kourou, French Guiana. LISA Pathfinder is the first UK led European Space Agency (ESA) science mission since the launch of Giotto in 1985. 25)

- In the coming weeks, the science module will be integrated with its instrument called the LISA Test Package Core Assembly (LCA), which has been built by a consortium of European universities, institutes and companies led by Airbus Defence and Space, Germany. The spacecraft will then undergo a series of environmental tests to determine its suitability for use in space. These tests will include thermal and acoustic tests and mass properties tests.

• June 16, 2015: After more than 10 years of intense development, Airbus DS in Friedrichshafen has completed the main component that will be at the heart of the highly sensitive payload of the LISA Pathfinder mission – the LISA LTP (Technology Package) Core Assembly. The LTP has been integrated in the LISA Pathfinder spacecraft. - The LISA Pathfinder mission will pave the way for the Evolved Laser Interferometer Space Antenna (eLISA) gravitational wave observatory that, after its scheduled launch in 2034, will track down the highest-energy and most violent astrophysical events unfolding across the Universe. 26)

Figure 7: Photo of the LTP (LISA Technology Package), also referred to as the science module, prior to integration into the spacecraft (image credit: DLR, Airbus DS)

• June 18, 2015: The LISA Pathfinder spacecraft, science module and propulsion module, are currently in the test center at IABG (Industrieanlagen-Betriebsgesellschaft), Ottobrunn, Germany. The spacecraft is designed to measure how well we can isolate a macroscopic body from all external forces except gravity. If successful, it will open the door to a new breed of spacecraft that can observe the gravitational Universe. For astronomers, this will be as if they developed a new sense, providing access to a view of the Universe that is wholly different to what they can detect now via electromagnetic radiation. 27)

- Since arriving at IABG in March 2015, the two modules have been tested — most of the time separately. This has allowed engineers to run individual activities in parallel, increasing the efficiency and improving the schedule. These tests have confirmed that all the spacecraft's systems are working as expected. In addition, a fit-check has been performed with the launch vehicle adapter, which is used to attach the spacecraft on the Vega launcher.

- The LTP has been integrated into the science module in recent weeks. Next, the science module and propulsion module will be joined to form the 'Launch Composite', so called because this is the configuration in which they will be when launched.

- With the Launch Composite complete it will be time to begin the final tests. These include the acoustic test, which is carried out to make sure the modules can survive the intense noise generated by the launcher rocket engines in the first few seconds after ignition.

- The final activity prior to shipment to the launch site will be the 'mass property measurement' of the Launch Composite. The entire spacecraft will be precisely weighed to determine its mass, the center of gravity, and the moments of inertia, which will be used to calculate the flight trajectory.

• October 8, 2015: The LISA Pathfinder spacecraft has arrived at Europe's spaceport in Kourou, French Guiana. 29)

Figure 10: On Oct. 9, 2015, the LISA Pathfinder transport container is being opened in the high bay of the EPCU (Ensemble de Préparation de la Charge Utile) S5C building at the Centre Spatial Guyanais, in Kourou (image credit: ESA-CNES-Arianespace / Optique Vidéo du CSG - G. Barbaste) 30)

• On Nov. 16, 2015, the LISA Pathfinder spacecraft was encupsulated into the half-shells of the Vega rocket fairing at the Centre Spatial Guyanais in Kourou. 31)

Figure 11: LISA Pathfinder is being encapsulated within the half-shells of the Vega rocket fairing (image credit: ESA, Manuel Pedoussaut)

Figure 12: Vega VV06 upper composite being hoisted up to the top of the mobile gantry (image credit: ESA)

Spacecraft:

The LISA Pathfinder spacecraft is being built and integrated by EADS Astrium Ltd. of Stevenage, UK (contract award in June 2004). The spacecraft is comprised of the science spacecraft (science craft) and a separable propulsion module for apogee raising. The science spacecraft contains the two main instruments LTP and DRS, and is covered with one single fixed solar array. The mass of the science craft after arrival at the operational orbit is about 480 kg. The sciencecraft dimensions are: 2.1 m diameter and 1.0 m in cylinder height. The propulsion module has a mass of 1420 kg (including fuel). The total launch mass is about 1900 kg (S/C dimensions of 2.9 m in length and 2.1 m diameter). 33)34)

The extremely stable and lightweight structure of the science module of LISA Pathfinder is made of CFRP (Carbon Fiber Reinforced Plastic) sandwich panels and shells. The mating ring of the propulsion module is made of aluminum alloy. The structure will provide a stable mounting to LISA Pathfinder's gravitational sensor technology package (LTP and DRS), limiting deformations at the interface during flight to less than 1 x 10-8 m Hz-1/2 in the instrument's measurement bandwidth between 1 and 30 mHz.

RF communications: X-band links will be used for TT&C as well as for science data service functions. The ground interface consists of a single 35 m X-band antenna providing about 8 hours of communications per day. Spacecraft operations will be done at ESOC while science and technology service functions will be provided by ESAC.

Communications to the spacecraft will be performed in X-band through a network of ground stations, including Kourou, Maspalomas and Perth, during LEOP (Launch and Early Operations Phase).

A new class of X-band transponder, referred to as X2PND, has been designed and developed by TAS-I (Thales Alenia Space – Italia) for LISA Pathfinder and for the Gaia mission of ESA. The compact X2PND device has a mass of 3.1 kg (including the diplexer), achieved through a high degree of integration. 35)

Legend to Figure 16: The sine test is performed (spring 2011) on the spacecraft in launch configuration with the PRM (Propulsion Module) mated to the SCM (Science Module) on top. 36)

Figure 17: LISA Pathfinder is paving the way for a future large space observatory that ultimately will directly observe and precisely measure gravitational waves (image credit: Airbus DS) 37)

Launch: The LISA Pathfinder spacecraft was launched on December 3, 2015 (4:04:00 UTC) on a Vega vehicle (VV06, as one of the VERTA flights) from Kourou, Europe's spaceport in French Guiana. 38)39)

The spacecraft separated from the upper stage at 05:49 UTC. Controllers at ESOC (European Space Operations Center) in Darmstadt, Germany then established control. Over the next two weeks, the spacecraft itself will raise the orbit's highest point in six critical burns. LISA Pathfinder is expected to reach its operational orbit about 10 weeks after launch, in mid February. After final checks, it will begin its six-month scientific mission at the beginning of March 2016.

Orbit: A halo orbit about the Lagrangian point L1 will be the operational orbit (1.5 million km away from Earth in the direction toward the sun). This location helps to minimize disturbances from the Earth's gravity, magnetic field and the atmosphere.

The spacecraft and the propulsion module will be injected into a slightly elliptical parking orbit of about 200 km x 1600 km at an inclination of 63º. From there, it will use the propulsion module (with a series of apogee burns) to enter into a transfer orbit and reach eventually its operational orbit around L1. After the transfer orbit the propulsion module separates from the science spacecraft (Figure 18). The LISA Pathfinder spacecraft will be stabilized using the micro-Newton thrusters, entering a Lissajous orbit around L1 (500,000 km x 800,000 km orbit around L1). 40)41)

Following the initial on-orbit check-out and instrument calibration, the in-flight demonstration of the LISA technology will then take place. The nominal lifetime of the mission is 180 days; this includes the LTP operations (90 days), the DRS operations (60 days), and a period of joint operations (30 days) when the LTP will control the DRS thrusters.

The constant orientation in the Earth-Sun direction of the spacecraft will provide a stable thermal environment.

• February 5, 2018: The final results from LISA Pathfinder show that the satellite's technology meets the requirements for space-based gravitational-wave detection. While LIGO and Virgo have grabbed everyone's attention, researchers are quietly pushing the frontiers of gravitational-wave astronomy into space. In 2015, the European Space Agency launched LISA Pathfinder (LPF)—a spacecraft aimed at demonstrating the technology for a spaceborne gravitational-wave observatory known as LISA (Laser Interferometer Space Antenna). Following its first tests in 2016, LPF has now reported its final results. Experiments onboard the now-deactivated spacecraft have shown that the acceleration of two free-falling masses can be measured with the accuracy needed for LISA. 42)

- LISA will observe gravitational waves by measuring how they stretch and compress the distance between two free-falling masses located on different satellites. Such measurements require the test masses to be in almost perfect free fall, isolated from any internal or external forces but gravity. After the 2016 tests, the LPF team implemented a series of improvements in the equipment that protects the masses from spurious forces that might jostle them. For instance, they found ways to reduce the pressure of the gas around the masses, which could create viscous forces, and to suppress the effect of inertial forces deriving from the satellite's rotation. As a result, compared with the 2016 results, LPF has now improved its acceleration sensitivity by a factor larger than 3 for the frequency range over which LISA is designed to operate. The results bode well for LISA, which will be sensitive to gravitational waves at lower frequencies than those detected so far. Such low-frequency waves could be created by supermassive-black-hole mergers and possibly by the inflationary expansion of the early Universe. 43)

• January 24, 2018: ESA's LISA Pathfinder mission has been honored with the 2017 Space Technology Award of the American Astronautical Society. — LISA Pathfinder, launched in December 2015 and operated between March 2016 and June 2017, has paved the way for future missions by flight-testing the concept of gravitational wave detection from space.44)

- The awards were announced earlier this week as part of the society's annual awards bestowed for meritorious accomplishments during the previous year, or not previously honored by them. "AAS is excited to honor these individuals and programs for their remarkable achievements and accomplishments," said Carol Lane, AAS President. "Their work is truly deserving of recognition and we at AAS thank and congratulate all of the winners who have each advanced space flight and exploration."

- The Space Technology award, established to recognize an outstanding achievement in space technology, was given to the international LISA Pathfinder team for "delivering and operating a novel spacecraft that clearly demonstrated that a space-based observatory of gravitational waves is within our technical reach".

- Gravitational waves, ripples in the fabric of spacetime, are produced by the acceleration of massive objects and can be generated by a wide range of cosmic phenomena, from supernova explosions to neutron star binaries spiralling around each other, and pairs of merging black holes.

- LISA Pathfinder hosted two test masses, linked by a laser interferometer, at the core of one spacecraft, and employed several sets of micronewton thrusters to nudge the spacecraft and keep it centered around them. The mission demonstrated that it is indeed possible for the test masses to be virtually motionless with respect to each other.

- Such a noise-free environment is essential in order to measure any possible distortion caused by a passing gravitational wave, which affect the fabric of spacetime on the minuscule scale of millionths of a millionth of a meter over a distance of a million kilometers.

- The mission surpassed all expectations, setting the stage for the development of ESA's LISA mission, a space-based constellation of three spacecraft, with launch planned for 2034.

• July 24, 2017: With the push of a button, final commands for the European Space Agency's LISA Pathfinder mission were beamed to space on July 18, a final goodbye before the spacecraft was powered down. LISA Pathfinder had been directed into a parking orbit in April, keeping it out of Earth's way. The final action this week switches it off completely after a successful 16 months of science measurements. 45)

• On July 18, 2017 at 19:57 CEST (Central European Standard Time), LISA Pathfinder principal investigator Stefano Vitale sent the final command to the spacecraft, permanently shutting it down after having successfully demonstrated the technology to build ESA's future space observatory for gravitational waves. 46)

• July 13, 2017: The final days of the LISA Pathfinder mission are some of the busiest, as controllers make final tests and get ready to switch off the gravitational pioneer on July 18. — Following 16 months of scientific effort, LISA Pathfinder completed its main mission on 30 June, having demonstrated the technology needed to operate ESA's future LISA space observatory to study gravitational waves – ripples in spacetime predicted by Albert Einstein in his General Theory of Relativity. 47)

- The future LISA mission will comprise three spacecraft orbiting some 2.5 million km apart in a triangular formation, with their ‘test masses' isolated from all external forces bar gravity and linked by laser beams.

- With the required sensitivity fully proven by LISA Pathfinder, teams are now using the spacecraft's last days to conduct a series of technical tests on components and devices, making full use of every remaining minute.

- "These tests will give us a better grasp of the craft's behavior and provide valuable feedback to the manufacturers about the characteristics of their equipment, in both routine and unusual conditions," says spacecraft operations manager Ian Harrison.

- "The gravitational wave detectors work by measuring the changing separation of two cubes that are in free-fall. Changes in the spacecraft's state or any movement may interfere with the measurements, and we want to better understand these for the future mission."

- In addition to satellite movement, the delicate cubes on LISA Pathfinder can be influenced by variations in their environment, such as in temperature and magnetic interference.

- Engineers have commanded the craft to turn to assess thermal effects on its systems, particularly the micropropulsion system, from solar illumination.

- Repeating thermal tests previously performed on the ground will help to improve procedures for the future LISA mission.

- Other tests are analyzing the effect of magnetic interference, from the operation of pressure regulation valves in the cold-gas thruster system, on the spacecraft's magnetic momentum, external forces and test mass control.

- The teams have also been pushing the micropropulsion system and test-mass electrostatic sensing and control systems to their limits.

- Spacecraft performance data have been recorded since the time of launch in December 2015 up to these last experiments, to determine the rate of hardware degradation in the harsh environment of space.

- Results from this test series will be available to European hardware manufacturers for incorporation into future designs. "These tests will help to eliminate variables that might influence the science results from future ESA missions, such as Euclid and LISA, and help reduce risk in their development," says flight director Andreas Rudolph.

- Ground teams are getting ready to ‘passivate' LISA Pathfinder, eliminating radio transmissions from the spacecraft and switching off most of the units.

- In April, the spacecraft used its thrusters over five days to nudge itself into a safe orbit around the Sun, minimizing any probability that it will return to the vicinity of Earth or Moon in the next 100 years, in line with ESA's requirement for space debris mitigation.

Figure 19: LISA Pathfinder mission controllers conducting flight operations at ESA/ESOC mission control center, Darmstadt, Germany, on 27 June 2017. After two years in orbit, LISA Pathfinder has already achieved much more than originally planned: to test in flight instruments that are enormously precise and to demonstrate that the technology will be capable of detecting gravity waves when used in the future full-scale LISA mission. With its prime technology demonstration mission complete, the satellite's final days will be dedicated to testing a number of operational procedures that could not be accommodated during its active mission. The team are now preparing the satellite for its final disposal maneuvers and shut-down, set for 18 July (image credit: ESA)

- Launched on 3 December 2015, ESA's LISA Pathfinder started its science mission in March 2016, shortly after the announcement of the first direct detection of gravitational waves – ripples in the fabric of spacetime.

- Since then, with two more observations of gravitational wave signals from merging black holes obtained using ground-based experiments – the latest one announced this month – it is clear that gravitational-wave astronomy has become a reality.

- Gravitational waves are produced by the acceleration of massive objects and can be generated by a wide range of cosmic phenomena, from supernova explosions to neutron star binaries spiralling around each other and pairs of merging black holes.

Legend to Figure 20: The collision of two black holes – an event detected for the first time ever by the LIGO (Laser Interferometer Gravitational-Wave Observatory) – is seen in this still from a computer simulation. LIGO detected gravitational waves, or ripples in space and time, generated as the black holes merged. The simulation shows what the merger would look like if we could somehow get a closer look. Time has been slowed by a factor of 100. The stars appear warped due to the strong gravity of the black holes.

- The signals detected so far have a frequency of around 100 Hz, corresponding to the coalescence of two stellar mass black holes, but gravitational waves span a much broader spectrum. To fully exploit this new window on the cosmos, it is crucial to be able to detect gravitational waves at low frequencies, between 0.1 mHz and 1 Hz, which are released by the mergers of the supermassive black holes sitting at the center of galaxies – these waves can only be observed from space.

- As LISA Pathfinder approaches the end of its successful technology demonstration mission, ESA's Science Program Committee has selected today the Laser Interferometer Space Antenna (LISA) as the third large mission (L3) in ESA's Cosmic Vision plan. LISA is a space-based observatory of gravitational waves consisting of a constellation of three spacecraft, with launch planned for 2034.

- In the LISA mission concept, six test masses will fly, two on each spacecraft. Each pair of test masses will be located at the end of the arm of the constellation, and will be linked to the others over millions of kilometers via lasers.

- The test masses must be placed in the most precise freefall that can be achieved, isolated from all external and internal forces except gravity, in order to measure any possible distortion caused by a passing gravitational wave. These waves affect the fabric of spacetime on the minuscule scale of millionths of a millionth of a meter over a distance of a million kilometers.

- "The whole point of LISA Pathfinder was to validate the technology for LISA, which requires test masses to be kept motionless to unprecedented levels of accuracy," says Paul McNamara, LISA Pathfinder Project Scientist at ESA. "This seemed so difficult and could not be tested on the ground, so we had to send the laboratory to space."

• On December 7, 2016, LISA Pathfinder started the extended phase of its mission, an additional six months during which scientists and engineers will push the experiment to its limits in preparation for ESA's future space observatory of gravitational waves. 49)

- Following completion of the DRS operations, the extended mission of LISA Pathfinder began on 7 December 2016, at 09:00 CET (08:00 UTC). It will last until 31 May 2017, making use of both the LTP and DRS payloads.

- "So far, we've been busy demonstrating the performance of LISA Pathfinder, which has been steadily improving as time went by," says Paul McNamara, LISA Pathfinder Project Scientist at ESA, "but now we can spend the next six months learning everything we need to know to build and operate a gravitational-wave observatory in space."

- Last October, ESA issued a call inviting European scientists to propose concepts for the third large mission (L3) in its Cosmic Vision plan, which will be a space observatory to study The Gravitational Universe. The selection is expected to take place in the first half of 2017, with a preliminary internal study phase planned for later in the year. 50)

- The future observatory will detect gravitational waves with frequencies from 1 Hz down to 0.1 mHz. These are about a hundred to a million times lower than the frequencies of waves that can be measured with ground-based experiments like the LIGO ( Laser Interferometer Gravitational-Wave Observatory), which obtained the first direct detection of gravitational waves in September 2015.

- During the extended mission of LISA Pathfinder, the team will run a series of long duration experiments to better characterize the mission performance especially at the lowest frequencies that will be probed by the future observatory. "We are thrilled to be pushing the limits of LISA Pathfinder, a unique physics laboratory in space giving us confidence that we can definitely build a space-borne observatory of gravitational waves", says Oliver Jennrich, LISA Pathfinder deputy mission scientist and L3 study scientist at ESA.

- One of the operations that will be attempted in the coming weeks concerns the station-keeping maneuvers that mission operators have been regularly conducting to keep the satellite on its operational orbit. LISA Pathfinder orbits around L1, but if left unattended, it would slowly drift away from the Lagrangian point under the gravitational pull of Earth. To avoid that, it is sufficient to fire the micro-newton thrusters once every one to two weeks.

- Between 25 December and 14 January, however, the team decided to apply no correction maneuvers. This will allow the scientists to run uninterrupted experiments for almost three weeks, exploring what happens in the range of very low frequencies that are of interest to detect gravitational wave from space.

- Another experiment concerns slightly higher frequencies, around 1–60 mHz. At these frequencies, the main source of disturbance seems to be gas molecules that are present in the test mass enclosures and bouncing off the two cubes – an effect that has been reducing as more molecules are being vented into space.

- The team is now curious to see whether additional sources of noise are lurking underneath, something that will be important for the future L3 mission. One possible way of testing this entails simply waiting until most molecules are vented into space, but there is an alternative: to switch off many of the heaters on board, reducing the temperature by ten degrees, and thereby reducing the pressure inside the enclosure. The team will run this experiment in late January.

- These are some examples of the range of experiments that will be conducted during LISA Pathfinder's extended mission. Eventually, at the end of the mission, the spacecraft will be gently pushed towards a heliocentric orbit.

• November 16, 2016: The ST7-DRS (Space Technology 7 Disturbance Reduction System), also referred to as DRS, is a system of thrusters, advanced avionics and software managed by NASA/JPL. As of Oct. 17, the system had logged roughly 1,400 hours of in-flight operations and met 100 % of its mission goals. The objective of the ST7-DRS thruster system is to hold Pathfinder as perfectly still as possible. This allows the spacecraft to test technologies used in the detection of gravitational waves, whose effects are so miniscule that it requires extreme steadiness to detect them. 51)

- Balancing all the disturbances on the spacecraft allows Pathfinder's instruments to stay in near-perfect free fall. This lays the groundwork for a future Pathfinder-type mission, which will need this kind of stability to cancel out any force other than the subtle tug of gravitational waves, produced by supermassive objects like black holes.

- DRS is a system of eight thrusters positioned on either side of the Pathfinder spacecraft. Each thruster emits microscopic liquid droplets called a colloid electrospray, which are created and charged through an electric field. These ionized droplets are accelerated by a second electric field with an opposite charge, which pushes them out of the thruster. The force of that reaction provides the "thrust" that steadies the spacecraft.

• On October 7, 2016, ESA's LISA Pathfinder Science Archive opened its virtual gates to the world. It contains data collected by the satellite during the mission's first few months, covering the nominal operations phase of the LTP (LISA Technology Package) – the European payload on LISA Pathfinder. 52)

- After the commissioning phase, science operations with the LTP payload started on 1 March and lasted until 25 June, 2016. The baton was then passed to the DRS (Disturbance Reduction System), an additional experiment provided by NASA on the LISA Pathfinder satellite, that is currently taking measurements. Operations with the LTP will start again in November, for seven months of the extended mission.

- First results based on just two months of science operations showed that LISA Pathfinder exceeded expectations, as the two test masses are falling freely through space under the influence of gravity alone, unperturbed by other external forces, to a precision more than five times better than originally required (Figure 22).

Legend to Figure 22: Results based on just two months of science operations on ESA's LISA Pathfinder show that the mission has demonstrated the technology needed to build a space-based gravitational wave observatory.

At the heart of the spacecraft, two identical, 2 kg, 46 mm gold–platinum cubes are falling freely through space under the influence of gravity alone, unperturbed by other external forces, to a precision more than five times better than originally required.

The two cubes are almost motionless with respect to each other, with a relative acceleration lower than ten millionths of a billionth of Earth's gravitational acceleration, g. The LISA Pathfinder team measured the remaining forces acting on the test masses, and identified three main sources of noise, depending on the frequency.

At the lowest frequencies probed by the experiment, below 1 mHz (on the left in this graph), the scientists measured a small centrifugal force acting on the cubes. This is caused by a combination of the shape of LISA Pathfinder's orbit and the effect of the noise in the signal of the startrackers used to orient it.

The contribution of the centrifugal force to the relative acceleration of the two test masses has been subtracted in this graph, and the source of the residual noise after subtraction is still being investigated.

At frequencies of 1–60 mHz (at the center), control over the test masses is limited by gas molecules bouncing off the cubes: a small number of them remain in the surrounding vacuum. This effect was seen to be reducing as more molecules were vented into space, and is expected to improve in the following months.

At higher frequencies, between 60 mHz and 1 Hz (on the right), LISA Pathfinder's precision is limited only by the sensing noise of the optical metrology system used to monitor the position and orientation of the test masses. Nevertheless, the performance of this system has already surpassed the level of precision required by a future gravitational-wave observatory by a factor of more than 100. The cause of the spike around 70 mHz is still under investigation.

Placing test masses in the purest freefall ever achieved is crucial to building a spaceborne observatory of gravitational waves – these are oscillations in the fabric of spacetime, moving at the speed of light and caused by the acceleration of massive objects.

The demonstration of the LISA Pathfinder's key technologies opens the door to the development of a large space observatory capable of detecting gravitational waves emanating from a wide range of exotic objects in the Universe.

• On June 25, 2016, the LTP (LISA Technology Package) on LISA Pathfinder, completed its nominal operations phase, passing the baton to the DRS (Disturbance Reduction System), an additional experiment provided by NASA. This won't be the last time the European experiment is run – the recently approved mission extension will see the LTP back in action for seven months starting in November 2016. 53)

- After over three months of outstanding scientific experiments, the first operations phase of the LISA Pathfinder mission is coming to an end. The conclusion of this part of the mission is foreseen for 08:00 UTC on 25 June 2016.

- After two weeks of commissioning, the operations phase of the DRS will last until the end of October. The LTP team will return for one week in early August to continue the long-term monitoring of their experiment and to facilitate some cross-calibration with the DRS experiment.

- An extended mission, approved by ESA's Science Program Committee at their 21-22 June meeting, will begin on 1 November, for seven months. During this period the team will further investigate the performance of the LTP at low frequencies – of particular interest in the context of a future space-based gravitational-wave observatory – as well as testing some experimental operational modes.

• June 7, 2016: Results from only two months of science operations show that the two cubes at the heart of the spacecraft are falling freely through space under the influence of gravity alone, unperturbed by other external forces, to a precision more than five times better than originally required. 54)55)

- In a paper published on June 7, 2016 in Physical Review Letters, the LISA Pathfinder team show that the test masses are almost motionless with respect to each other, with a relative acceleration lower than ten millionths of a billionth of Earth's gravity. 56)

- The demonstration of the mission's key technologies opens the door to the development of a large space observatory capable of detecting gravitational waves emanating from a wide range of exotic objects in the Universe.

- Hypothesized by Albert Einstein a century ago, gravitational waves are oscillations in the fabric of spacetime, moving at the speed of light and caused by the acceleration of massive objects. They can be generated, for example, by supernovas, neutron star binaries spiralling around each other, and pairs of merging black holes.

- Even from these powerful objects, however, the fluctuations in spacetime are tiny by the time they arrive at Earth – smaller than 1 part in 100 billion billion.

- Sophisticated technologies are needed to register such minuscule changes, and gravitational waves were directly detected for the first time only in September 2015 by the ground-based Laser Interferometer Gravitational-Wave Observatory (LIGO).

- This experiment saw the characteristic signal of two black holes, each with some 30 times the mass of the Sun, spiralling towards one another in the final 0.3 seconds before they coalesced to form a single, more massive object.

- The signals seen by LIGO have a frequency of around 100 Hz, but gravitational waves span a much broader spectrum. In particular, lower-frequency oscillations are produced by even more exotic events such as the mergers of supermassive black holes.

- With masses of millions to billions of times that of the Sun, these giant black holes sit at the centers of massive galaxies. When two galaxies collide, these black holes eventually coalesce, releasing vast amounts of energy in the form of gravitational waves throughout the merger process, and peaking in the last few minutes.

- To detect these events and fully exploit the new field of gravitational astronomy, it is crucial to open access to gravitational waves at low frequencies between 0.1 mHz and 1 Hz. This requires measuring tiny fluctuations in distance between objects placed millions of kilometers apart, something that can only be achieved in space, where an observatory would also be free of the seismic, thermal and terrestrial gravity noises that limit ground-based detectors.

- LISA Pathfinder was designed to demonstrate key technologies needed to build such an observatory. A crucial aspect is placing two test masses in freefall, monitoring their relative positions as they move under the effect of gravity alone. Even in space this is very difficult, as several forces, including the solar wind and pressure from sunlight, continually disturb the cubes and the spacecraft.

- Thus, in LISA Pathfinder, a pair of identical, 2 kg, 46 mm gold–platinum cubes, 38 cm apart, fly, surrounded, but untouched, by a spacecraft whose job is to shield them from external influences, adjusting its position constantly to avoid hitting them.

- "LISA Pathfinder's test masses are now still with respect to each other to an astonishing degree," says Alvaro Giménez, ESA's Director of Science. "This is the level of control needed to enable the observation of low-frequency gravitational waves with a future space observatory."

- The mission started operations on 1 March, with scientists performing a series of experiments on the test masses to measure and control all of the different aspects at play, and determine how still the masses really are. "The measurements have exceeded our most optimistic expectations," says Paul McNamara, LISA Pathfinder Project Scientist. "We reached the level of precision originally required for LISA Pathfinder within the first day, and so we spent the following weeks improving the results a factor of five."

- These extraordinary results show that the control achieved over the test masses is essentially at the level required to implement a gravitational wave observatory in space. "Not only do we see the test masses as almost motionless, but we have identified, with unprecedented precision, most of the remaining tiny forces disturbing them," explains Stefano Vitale of University of Trento and INFN, Italy, Principal Investigator of the LISA Technology Package, the mission's core payload.

- The first two months of data show that, in the frequency range between 60 mHz and 1 Hz, LISA Pathfinder's precision is only limited by the sensing noise of the laser measurement system used to monitor the position and orientation of the cubes. "The performance of the laser instrument has already surpassed the level of precision required by a future gravitational-wave observatory by a factor of more than 100," says Martin Hewitson, LISA Pathfinder Senior Scientist from Max Planck Institute for Gravitational Physics and Leibniz Universität Hannover, Germany.

- At lower frequencies of 1–60 mHz, control over the cubes is limited by gas molecules bouncing off them – a small number remain in the surrounding vacuum. This effect was seen reducing as more molecules were vented into space, and is expected to improve in the following months.

Legend to Figure 23: This plot shows the result of LISA Pathfinder's two-month experiment in drag-free flight, where the goal is to follow test masses as they fall through space affected only by gravity. LISA Pathfinder reduced non-gravitational forces on the test masses to a level five times better than the mission required and within 25% of the requirement for a future space-based gravitational wave detector. The cause of the spike around 0.07 Hertz is still under investigation. The line labeled "noise model" represents a simple physical model of the measured performance. It consists of a flat component, dominant at low frequencies, arising from residual gas molecules around the test mass and a rising component, dominant at higher frequencies, representing the limits of the instrument's ability to sense the motion of the test masses. This model explains the vast majority of the observed behavior, providing confidence that such models can be used to extrapolate from LISA Pathfinder to a full-scale future observatory.

- "We have observed the performance steadily improving, day by day, since the start of the mission," says William Weber, LISA Pathfinder Senior Scientist from University of Trento, Italy.

- At even lower frequencies, below 1 mHz, the scientists measured a small centrifugal force acting on the cubes, from a combination of the shape of LISA Pathfinder's orbit and to the effect of the noise in the signal of the startrackers used to orient it.

- While this force slightly disturbs the cubes' motion in LISA Pathfinder, it would not be an issue for a future space observatory, in which each test mass would be housed in its own spacecraft, and linked to the others over millions of kilometers via lasers.

- "At the precision reached by LISA Pathfinder, a full-scale gravitational wave observatory in space would be able to detect fluctuations caused by the mergers of supermassive black holes in galaxies anywhere in the Universe," says Karsten Danzmann, director at the Max Planck Institute for Gravitational Physics, director of the Institute for Gravitational Physics of Leibniz Universität Hannover, Germany, and Co-Principal Investigator of the LISA Technology Package.

- Today's results demonstrate that LISA Pathfinder has proven the key technologies and paved the way for such an observatory, as the third 'Large-class' (L3) mission in ESA's Cosmic Vision program.

• March 8, 2016: Following a long series of tests, ESA's LISA Pathfinder has started its science mission to prove key technologies and techniques needed to observe gravitational waves from space. Predicted by Albert Einstein a century ago, gravitational waves are fluctuations in the fabric of spacetime produced by exotic astronomical events such as supernova explosions or the merging of two black holes. 57)

- The scientific mission of LISA Pathfinder officially started on 1 March. Following a formal review of the commissioning period on 7 March, the mission was formally handed over from the ESA project and industrial teams that built it to the scientists who are now busy carrying out experiments on this unique gravity laboratory in space. 58)

- The results of LISA Pathfinder's precision experiments will pave the way towards the L3 mission in ESA's Cosmic Vision program, a future project that will be dedicated to investigating the gravitational Universe by means of a large spaceborne observatory.

- LISA Pathfinder cannot in itself detect gravitational waves – since the signal of gravitational waves is so tiny, the test masses would need to be millions of kilometers apart rather than the 38 cm available on board LISA Pathfinder. LISA Pathfinder must prove the ultra-high precision technology needed to make a test mass float freely in space. This will mean that any effects on its trajectory can only be the result of external gravitational forces. These test masses are two metal cubes which will be placed into gravitational freefall. 59)

• March 1, 2016: After , weeks of engineering tests, the LISA Pathfinder science team, based at ESOC in Darmstadt, Germany, is ready to start work, demonstrating key technologies to observe gravitational waves from space. There, the scientists are working closely with the spacecraft engineers to run experiments on this outstanding physics laboratory in space. Every day, the scientists will analyze the results from the previous day's experiments and plan the measurements to perform in the coming days. The operations phase will last six months, split between 90 days for the LTP (LISA Technology Package), and 90 days for the DRS (Disturbance Reduction System), an additional experiment provided by NASA/JPL (Jet Propulsion Laboratory). 60)

- Over the coming months, the scientists will perform several experiments, applying different forces to the masses and studying their response to assess how much their motion can deviate from actual freefall.

• On Feb. 22, 2016, the two cubes housed in the core of ESA's LISA Pathfinder were left to move under the effect of gravity alone – another milestone towards demonstrating technologies to observe gravitational waves from space. 61)

- One of the most delicate operations entailed releasing the two test masses from the mechanisms that kept them in place during ground handling, launch and cruise.

- First, the eight locking ‘fingers' pressing on the corners of the identical gold–platinum cubes were retracted on February 3. The cubes were then being held in position only by two rods, softly pushing on opposite faces.

Legend to Figure 24: On top is the solar array providing power to the payload and support systems, and shields its sensitive payload from the sun. Beneath the array is the white- and gold-hued science module, which carries the payload with the test masses and their electrode housings, the optical bench interferometer, and vacuum enclosure. All of these components fit neatly within the central cylinder, which in turn slots into the center of the science module. - The science module also contains support systems for LISA Pathfinder's scientific experiments, and carries micro-newton thrusters on its outer panels to adjust the position of the spacecraft. The propulsion module at the bottom of the exploded view helped LISA Pathfinder to reach its final orbital location.

• Feb. 16, 2016: ESA's LISA Pathfinder has released both of its gold–platinum cubes, nicknamed Jake and Elwood, and will shortly begin its demanding science mission, placing these test masses in the most precise freefall ever obtained to demonstrate technologies for observing gravitational waves from space. 62)

- As tests on the spacecraft and its precious payload continue, a major milestone was reached today. For the first time, the two masses – a pair of identical 46 mm gold–platinum cubes – in the heart of the spacecraft are floating freely, several millimeters from the walls of their housings. The cubes sit 38 cm apart linked only by laser beams.

- Throughout LISA Pathfinder's ground handling, launch, the burns that raised its orbit, and the six-week cruise to its work site, each cube was held firmly in place by eight ‘fingers' pressing on its corners.

- It will be another week before the cubes are left completely at the mercy of gravity, with no other forces acting on them. Before then, minute electrostatic forces are being applied to move them around and make them follow the spacecraft as its flight through space is slightly perturbed by outside forces such as pressure from sunlight.

- On 23 February, the team will switch LISA Pathfinder to science mode for the first time, and the opposite will become true: the cubes will be in freefall and the spacecraft will start sensing any motions towards them owing to external forces. Microthrusters will make minuscule shifts in order to keep the craft centered on one mass.

- Then the scientists will be in a position to run several months of experiments to determine how accurately the two freely-flying test masses can be kept positioned relative to each other, making measurements with the laser that links them.

• Feb. 3, 2016: Today, the lock fingers that kept the two test masses on LISA Pathfinder secure during the launch and cruise phase were successfully unlocked. As planned, the two cubes are still attached to the spacecraft via an additional mechanism that will hold them in place until mid February, as the teams carry on with the spacecraft and payload commissioning. 63)

• January 22, 2016: After a six-week journey, LISA Pathfinder arrived at its destination today, an orbit around Lagrangian Point L1, where it will soon start testing technologies crucial for exploring the gravitational Universe. LISA Pathfinder's arrival came after a final thruster burn using the spacecraft's hard-working propulsion module on 20 January. The small, 64-second firing was designed to slightly change its speed and just barely tip the craft onto its new orbit about L1. Since launch, the propulsion module raised the orbit around Earth six times, the last of which kicked it towards L1. 64)

- The propulsion module separated from the science section at 11:30 GMT today after the combination was set spinning for stability. The operations were monitored by the mission control and science teams at ESOC in realtime via the Agency's deep-space station at Malargüe, Argentina.

• January 21, 2016: The DRS (Disturbance Reduction System), managed by NASA/JPL, passed their functional tests on Jan. 10, 2016. The thrusters achieved their maximum thrust of 30 µN, equivalent to the weight of a mosquito. This level of precision is necessary to counteract small forces on the spacecraft such as the pressure of sunlight, with the result that the spacecraft and the instruments inside are in near-perfect free-fall. A mission to detect gravitational waves would need that level of stability. 65)

• January 11, 2016: While LISA Pathfinder is en route to its operational orbit, the science and engineering teams are testing the systems on the spacecraft. This week, they will begin to switch on elements of the science payload, including the laser that will be used to monitor the most precise free-fall motion ever obtained in space. 66)

- LISA Pathfinder has used its propulsion module to raise its orbit six times and embark on the path to its operational orbit around the Lagrange point L1, 1.5 million km from Earth in the direction of the Sun. After releasing the propulsion module on 22 January, the science module will enter its final orbit. Science operations will then begin on 1 March.

- During LISA Pathfinder's cruise to L1, teams from ESA and Airbus Defence and Space (the prime contractor) have been commissioning the spacecraft, verifying that all systems, subsystems and instruments function as expected. Just before Christmas, the cold gas micro-newton thrusters, which will be used to accurately adjust the spacecraft position by tiny shifts during the operations phase, were first activated. Last week, the colloidal micro-newton thrusters, provided by NASA-JPL as part of the Disturbance Reduction System, were switched on and tested.

- This week, the teams start commissioning the LTP (LISA Technology Package), switching on the payload computer and other electronics, testing the control unit of the caging mechanism that holds the test masses secure during launch and cruise, and verifying that the radiation monitor works well.

- On 13 January, they will switch on the laser, the first scientific component of the LISA Technology Package to be activated. During science operations, the laser will be used to feed two light beams to the interferometer, which will measure the position and attitude of the free-falling test masses to unprecedented accuracy.

Legend to Figure 25: The two gold cubes shown in this image are key to the LISA Pathfinder mission. Each of these electrode containers houses a gold-platinum test mass, visible in the cutaway on the right. LISA Pathfinder will monitor the two cubes as they enter free-fall motion using a high-precision laser interferometer.
The optical bench interferometer is shown between the two masses. This instrument is made from a 20 cm by 20 cm block of ZERODUR ceramic glass, and has a set of 22 mirrors and beam-splitters bonded to its surface that direct laser beams. These beams will allow scientists to precisely measure the cubes' motion, position, and orientation without touching them. In this way, LISA Pathfinder will perform the first high-precision laser interferometric tracking of orbiting bodies in space.

- The commissioning of the spacecraft and instruments will continue until the end of February. During this period, the test masses will be released, in a two-step process, from the mechanisms that have been holding them secure in position during the launch and cruise phase.

• Dec. 11, 2015: Europe's Vega light launcher is entering its commercial life boasting a flawless record and an impressive set of capabilities for a wide range of missions. Vega scored its sixth straight success with the launch of ESA's LISA Pathfinder scientific craft earlier this month, having already lofted payloads for Earth observation, space engineering and exploration. Operator Arianespace has now taken over full responsibility for Vega's commercial exploitation at Europe's Spaceport in Kourou, French Guiana. 67)

Sensor/experiment complement: (LTP, FEEP, DRS)

The sensor complement comprises two packages: the LTP (LISA Technology Package) and the DRS (Disturbance Reduction System). Both will test the key technology of drag-free control by means of proof masses. 68)

Metrology concept: In LISA Pathfinder, the traditional distinction between spacecraft and payload disappears. The instrument really involves the entire spacecraft. It is fair to state that LISA Pathfinder implements a "formation flight" of three orbiting bodies, namely the spacecraft and the pair of test-masses. This formation flight is implemented using one of several variants of the basic "drag-free" control scheme: One of the test-masses is in pure free-fall in all translational degrees of freedom (x, y, and z in Figure 26) and no force is purposely applied to it. The spacecraft follows this first test-mass within a standard drag-free control scheme in all translational degrees of freedom. The second test-mass is free along y and z and the spacecraft can follow this by using rotation around z and y, respectively. 69)70)71)72)

The LISA Pathfinder measurement scheme, with the separate high-resolution optical readout of test-mass motion relative to the spacecraft, allows test-mass to test-mass tracking with accuracy unspoiled by the spacecraft motion even for test-masses located in different spacecraft. Indeed, as in LISA, one can track one test-mass relative to its hosting spacecraft and then one spacecraft relative to the other one and then reconstruct the test-masses' relative motion by adding up these three measurements. Thus LISA Pathfinder demonstrates the possibility of undertaking high resolution geodesy with test-mass to test-mass tracking.

LTP (LISA Technology Package)

LTP is a payload developed for ESA by the European scientific community using national funding (Table 3). Institutes and industries of the following countries are involved: France, Germany, Italy, Spain, Switzerland, The Netherlands, and the United Kingdom. Airbus DS, formerly EADS Astrium GmbH (Friedrichshafen, Germany) is the prime contractor for LTP payload integration (subsystem deliveries from national teams). LTP co-Principal Investigators are Stefano Vitale of Trento University, Italy and Karsten Danzmann of the Max-Planck-Institut für Gravitationsphysik (Albert Einstein Institut) in Hannover, Germany.

Table 3: Responsibilities in the manufacture of the LISA Technology Package

The LTP concept on LISA Pathfinder represents one arm of the LISA constellation interferometer, in which the distance between the two proof masses is reduced from 5 million km to about 35 cm. As in LISA, the proof masses fulfil a double role: they serve as mirrors for the interferometer and as inertial references for the drag-free control system.

• To demonstrate the feasibility of performing laser interferometry in the required low-frequency regime with a performance as close as possible to 10-12 ms-2 Hz-1/2 in the frequency band 10-3 to 10-1 Hz , as required for the LISA mission

• To assess the longevity and reliability of the capacitive sensors, thrusters, lasers and optics in the space environment.

• The final objective of LISA Pathfinder is to confirm the overall physical model of forces that act on a test-mass in interplanetary space.

The LTP measurement scheme is shown in Figures 27 and 28. LTP contains the two (partially) free-floating TM (Test Masses), each surrounded by a sensor cage. Each cage is rigidly attached to the optical bench, i.e. the spacecraft. The distance between the two test masses is measured with an optical metrology system (laser interferometer) along the sensitive x-axis which is the nominal line of connection between the two test masses. The proof (or test) masses are made of a gold-platinum, low magnetic susceptibility alloy, have a mass of m = 1.96 kg and are separated by a nominal distance of 376 mm. The proof masses for LISA Pathfinder are the same as those foreseen for LISA.

Metrology system (Figure 28): The optical bench of LTP in LISA Pathfinder uses a total of four interferometers:

- 1) one to measure the distance between the proof masses,

- 2) one to measure the distance of one of the proof masses with respect to the optical bench,

- 3) and 4) two interferometers to assess the residual frequency noise of the laser.

The optical bench is made from low-expansion glass (Zerodur), the optical components are attached using hydroxy-catalysis bonding, a technique developed for the GP-B (Gravity Probe-B) mission to ensure the long-term stability of the components' position.

Once in orbit the residual differential acceleration noise of the proof masses is measured. In order to be able to measure differential acceleration, the sensitive axes of the two test-masses have to be aligned. This requires the development of a capacitive suspension scheme that carries one or both test-masses along with the spacecraft, including along the measurement axis. In LISA Pathfinder (LPF), the optical metrology system essentially makes two measurements; the separation of the test masses, and the position of one test mass with respect to the optics bench. The latter measurement is identical to the LISA local measurement interferometer, thereby providing an in-flight demonstration of precision laser metrology directly applicable to LISA. - Hence, this minimal instrument concept of LTP on LISA Pathfinder is deemed to contain the essence of the construction procedure needed for LISA and thus to demonstrate its feasibility.

From a control point of view, the major task of the drag-free system is the stabilization of the test mass relative coordinates (12 DoF) and the spacecraft attitude (3 DoF). In total these are 15 DoF to be controlled. In the "science" mode, this must be accomplished while minimizing any non-gravitational acceleration along the "sensitive axis". See Figure 27.

In LISA and in LPF (LISA Pathfinder), charging by cosmic rays is a major source of disturbance, thereby each test-mass carries a non contacting charge measurement and neutralization system based on UV photoelectron extraction. An in-flight test of this device is then obviously a key element of the overall LPF test.

Each proof (test) mass is surrounded by a set of electrodes that are used to read out the mass position and orientation relative to the spacecraft. This measurement is obtained as the motion of the proof mass varies the capacitances between the electrodes and the proof mass itself. The same set of electrodes is also used to apply electrostatic forces to the proof masses. Differential capacitance variations are parametrically read out by a front-end electronics composed of high accuracy differential inductive bridges excited at about 100 kHz, and synchronously detected via a phase sensitive detector.

Each proof mass, with its own electrode housing, is enclosed in a high vacuum chamber which is pumped down to 10-5 Pa by a set of getter pumps. The laser interferometer light passes through the vacuum chamber wall through an optical window. The free-falling system formed by one test mass, its electrode housing, the vacuum enclosure and the other subsystems is referred to as the GRS (Gravity Reference Sensor).

Within the LTP, a key element for suppressing the force disturbance is that the proof masses have no mechanical contact to the spacecraft. In addition, as forces may depend on the position of the proof masses within the spacecraft, this is kept as fixed as possible.

To fulfil both of these apparently conflicting requirements, the spacecraft actively follows the proof (or test) mass located within it in a closed-loop control scheme referred to as drag-free control. The position of the proof mass relative to some nominal origin is measured by means of the gravitational reference sensor. A high gain control loop tries to null this error signal by forcing the spacecraft to follow the proof mass. In order to produce the necessary force on the spacecraft, the control loop drives a set of micro-thrusters. 79)

1) Heterodyne measurement of the relative position of the proof masses along the sensitive axis

2) Heterodyne measurement of the position of one of the proof masses (proof mass 1) relative to the optical bench

3) Differential wave-front sensing of the relative orientations of the proof masses around the y-axis and the z-axis

4) Differential wave-front sensing of the orientation of proof mass 1 around the y-axis and the z-axis. Sensitivities at mHz (milli Hertz) frequency are in the range of 10 pm (1 picometer = 10-12 m) Hz-1/2 for displacement and of 10 nrad Hz-1/2 in rotation.

A diode-pumped, monolithic Nd:YAG non-planar ring oscillator (wavelength 1.064 µm) is used as the light source for the heterodyne interferometry. To obtain the necessary frequency shift, the beam coming from the laser is split and each partial beam is sent through an AOMU (Acousto-Optical Modulator Unit). The light is then delivered to the optical bench by a pair of optical fibers and fiber injectors. Quadrant photo-diodes are used for the detection of the interferometric signal, permitting the measurement of yaw and pitch of the proof masses with respect to the sensitive axis.

Legend to Figure 32: The partly transparent view reveals the two proof masses: 46 mm large cubes of a gold/platinum alloy, housed in individual vacuum cans. The cubes serve both as mirrors for the laser interferometer (red lightpaths) and as inertial references for the drag-free control system of the spacecraft.

The LTP subsystems are:

• ISS (Inertial Sensor Subsystem)

• CMA (Caging Mechanism Assembly)

• CMD (Charge Management Device)

• FEE (Front End Electronics)

• OBI (Optical Bench Interferometer)

• RLU (Reference Laser Unit)

• AOMU (Acoustic Optic Modulator Unit)

• FEEP (Field Effect Electric Propulsion) with µN thrusters

• DDS (Data and Diagnostic Subsystem)

• OBC (On-Board Computer)

• DFACS (Drag-Free and Attitude Control Subsystem)

The LTP instrument package has a mass of about 125 kg, power consumption of about 150 W, and a size of: 64 cm x 38 cm x 38 cm.

Figure 33: Photo of the LPT during the acoustic tests at ESTEC in September 2008 (image credit: ESA)

Legend to Figure 33: This photograph shows two dummies of the two vacuum chambers which will contain the proof masses in the electrodes housing boxes, and the connecting optical bench.

The LTP (LISA Technology Package) instrument consists of two main functional subsystems, namely the ISS (Inertial Sensor Subsystem), also known as ISH (Inertial Sensor Head), and the OMS (Optical Metrology Subsystem) as shown in Figure 41, which are controlled by the DDS ( Data & Diagnostics Subsystem).

• The ISH is providing all technical means necessary to bring the two LTP TM (Test Masses) into orbit and then - steered by DFACS algorithms - to control the TM attitude and position through electrostatic actuation and suspension so that one TM is freely falling in direction towards the other. The ISS comprises six main subsystems (Ref. 21):

There are two ISH (Inertial Sensor Heads) on either end of the LTP (LISA Technology Package), mounted to a Zerodur support structure, to which the optical bench is also fixed (Figure 40). Each ISH carries a test mass, which will be free floating once the spacecraft is on orbit. Each of these test masses will be floating inside a set of parallel electrodes, which measure the distance from the mass to its walls. The distance between the two test masses will be measured using a laser interferometer, to confirm that the masses can be maintained in a genuinely free-floating condition inside the spacecraft and that the distance between them can be measured with sufficient accuracy and noise level. These elements will be vital for any future gravitational wave detection mission, which will need to place the masses much farther apart in space. 81)

Creating the correct environment for the test masses poses several challenges: the structure must be perfectly aligned down to micron level, even when it is subjected to the extreme acoustic and mechanical environment of launch. The vacuum enclosure, which is almost 43cm tall and almost 18cm in diameter, must avoid magnetic components: for this reason it was constructed from titanium instead of the more conventional steel, and special feedthroughs were required for UV fibers and electrical cables. The ISH must be constructed from low outgassing materials, to attain a high vacuum level, and this vacuum level must be preserved up to launch. The Caging and Venting Mechanism, which functions as a launch lock, ensures that any tiny air pockets – "virtual leaks" from within the environment – are vented to space, preserving the vacuum.

The mechanical performance of the ISH during launch was verified by a series of tests. The campaign included vibration testing in three planes, carried out by CGS (Carlo Gavazzi Space) at the Centro Technica in Milan, Italy. After a functional test of the mechanism by which the test mass is handed over from the CSV to the GPRM, the ISH was first subjected to swept-sine vibrations at low level, to determine its characteristics and to reveal any mechanical resonances not predicted by the design models. The ISH was then subjected to random vibrations at qualification level – these tests verify the overall design of the ISH. This was followed by a random vibration life test (three cycles at acceptance level – to verify that the performance satisfies the specifications).

The integration of all the components of the ISH with perfect alignment, and the successful completion of the tests mark a major milestone for LISA Pathfinder. It is the first time that a heavy test mass inertial sensor has been assembled and tested successfully anywhere in the world. All flight model elements of the ISH have been delivered and work continues on the LISA Technology Package, due for completion in November 2014, after which it will be delivered and integrated into the spacecraft.

• The OMS serves as a high precision optical sensor of the differential movement of the two TM and of the movement of one of the TM with respect to the LPF SCM (Science Module). It is based on heterodyne Mach-Zender interferometry allowing for high precision measurements of TM position and attitude, e.g. intrinsically reaching the range of 6 x10-12 m/Hz1/2 x [1+(f/3 mHz)2] for 3 - 30 mHz in case of position sensing.

Data acquisition, conditioning and phase measurement is performed by the interferometer front-end electronics, based largely on field programmable gate arrays (FPGA). The final processing and retrieval of the position signals from the phase measurements is performed by the LTP payload computer. 84)

Legend to Figure 44: The optical bench seen here was developed for ESA by the University of Glasgow and University of Birmingham in the UK. The cylinder containing the test mass was developed by CGS in Milan, Italy, and the integration is now taking place at Airbus Defence and Space in Friedrichshafen, Germany.

Figure 44 provides an intimate view of a key part of the payload of ESA's LISA Pathfinder satellite, which will be the ‘stillest' ever flown in space – in fact, the distant-orbiting spacecraft is set to become the single most stable place in the Solar System.

These transparent ‘gravestones' are made of fused silica glass, used to split then recombine a pair of laser beams. The glass elements are aligned down to a few thousands of a millimeter onto the supporting optical bench, made from ultra-low expansion Zerodur glass. This laser system will measure the very slightest movements of a pair of gold–platinum test masses, right down to subatomic scale precision.

One test mass is placed inside a ‘capacitive sensing' housing within the cylinder behind the optical bench, visible in the picture, and the other will be in an identical cylinder - due to be placed ahead of it. In space, these test masses will float freely within the spacecraft, which will maneuver itself to keep them away from the housing walls.

The aim is to allow the test masses to be subject only to the underlying force of gravity, mapping the very slight curvature of local space-time.

The L1 (Lagrangian Point 1) location of the LISA Pathfinder in space will enable the spacecraft to minimize the effects of external perturbations. In addition, the spacecraft itself actively compensates for other forces acting upon it – even firing µN thrusters to compensate for the tiny but significant ‘push' of sunshine.

FEEP (Field Effect Electric Propulsion) Subsystem:

The LISA Pathfinder MPS (Micro-thrust Propulsion Subsystem) is comprised of three main subsections called MPA (Micro-Propulsion Assemblies), each one consisting of one FCA (FEEP Cluster Assembly), one PCU (Power Control Unit), and one NA (Neutralizer Assembly) as shown in Figure 45. The main features of the propulsion system are: 86)87)88)89)90)

• It is able to produce stable thrust levels ranging from 0.1 µN to 150 µN

• It is able to produce thrust with a resolution capability better than 0.1 µN and time response better than 200 ms for any specified thrust step in the required thrust range

• Thrust noise, as measured indirectly through electrical parameters and through direct beam sampling, is compatible with the requirement for proper DFACS operation

• Once deployed and initialized in orbit, it has no moving parts, nor gas leaks that could result in spacecraft disturbance

• The thruster does not need ferromagnetic materials, and the magnetic disturbance on the test mass can be prevented by adequate design rules.

Each MPA is mounted on the spacecraft at 120º with respect to the others with each FCA allocating four FEEP thrusters. The four FEEP thrusters, commanded individually and working in hot redundancy, are mounted in such a way that the relevant thrust vector directions allows to maximize the spacecraft positioning in all spacecraft directions (each thrust vector has 45º of nominal azimuth angle and 30º of nominal elevation). The PCU is allocated inside the spacecraft while the FCA and NA (Neutralizer Assembly) are mounted externally.

The NA (Neutralizer Assembly) consists of a self-contained unit of two Neutralizer unit mounted on a support structure with any necessary interfaces and support bracket (mechanical, thermal and electrical). The neutralizer is necessary to nullify the spacecraft unbalance due to ion thruster operation. The neutralization function is implemented by means of cold redundant hardware.

The PCU (Power Control Unit) consists of a self-contained electronic unit mounted on a support structure with any necessary interfaces and support bracket (mechanical, thermal and electrical). The PCU interfaces the spacecraft (Power and TC/TM tasks) and provides power and control to both FEEP Cluster and Neutralizer assemblies. The HV interconnection box and relevant harness is part of this equipment.

The LISA Pathfinder FEEP Subsystem has been developed to embark two different FEEP thrusters technologies currently under qualification in Europe: one using slit-shaped emitter with Cesium as propellant and the second using a needle-shaped emitter with Indium as propellant.

• In the slit-shaped emitter design (developed by Alta SpA, Italy), field emission is generated applying the intense electric field on the liquid metal (Cesium), which is heated above its melting point (≈ 29ºC), inside the edge of two sharp blades so forming the emitter slit. The equilibrium between the surface tension and the electric field strength forms the so-called Taylor cone on the surface with a jet protruding due to space charge. Atoms are then ionized at the tip of the jet and accelerated out by the same field that created them. This configuration allows to form several field emission sites (Taylor cones) along the length of the slit directly proportional to the commanded thrust. With this design approach the thrust range can be extended simply increasing the length of the slit. 91)92)

The FT-150 FEEP (Field Emission Electric Propulsion) microthruster is designed for extremely fine positioning and attitude control applications. It generates thrust by ejecting cesium ions at about 100 km/s of speed with a noise level lower than the threshold of the nano-balance used in the direct measurement, about 0.1 µN/√Hz, in the 10 mHz to 10 Hz range. Ions are extracted from the emitter tip and accelerated by the electric field created between the emitter and the accelerator electrode placed in front of it. The total voltage applied to the electrodes is between 7 kV and 13 kV. The specific impulse of the FT-150 FEEP microthruster is substantially larger than the typical range of ion thrusters, varying between 3000 s and 4500 s depending on operating conditions. Performance was verified by testing throughout a thrust range from 0.1 µN to 150 µN. Alta SpA (Italy) carried out the development of the FT-150 FEEP microthruster with the aim to fulfill the requirements of the fundamental physics missions LISA and Microscope, as well as those of the LISA technology demonstrator LISA Pathfinder. 93)

Parameter

Value

Remarks

Nominal power

6 W

@ 100 µN of thrust

Thrust range

0.1 to 150 µN

Thrust resolution

Below 100 nN

Thrust accuracy

± 1.6 µN at max thrust
± 0.4 µN up to 4 µN

Thrust response time

50-150 ms

Thrust noise

< 0.1 µN/√Hz

Below nano-balance detection threshold

Specific impulse

About 6000 s (depending on emitter voltage)

Total impulse capability

> 5000 Ns

2631 Ns demonstrated by endurance test

Thruster dry mass

~1400 g

Propellant mass

92 g

Per thruster

Table 4: FT-150 FEEP microthruster performance data

• In the needle-shaped design [developed by ARC (Austrian Research Centers GmbH) of Schreibersdorf, Austria], field emission is generated applying the intense electric field on the liquid metal (Indium), which is heated above its melting point (≈ 156ºC), on a needle-shaped configuration. In this case, due to limited thrust capability of a single needle (it allows a single site emission only), a cluster of several needles need to be used to form a thruster with suitable thrust range (e.g. nine to cover LISA Pathfinder requirements). The indium happens to be less reactive than cesium, hence it can be more easily managed during all phases of AIT and mission. 94)95)96)

The two techniques are equivalent as far as the propulsion performances are concerned; however, the slit cesium design requires less electrical power to liquefy the propellant with respect to needle-indium due to lower melting temperature (29ºC against 156ºC). The mass budget is similar for thrust values of about 100 µN and is resulting favorable for the needle in case of lower thrust range and, conversely, beneficial for slit in case of higher thrust range.

Legend to Figure 47: On the left side the extractor electrode [2], the focusing electrode [3] and the cover-plate [4]. On the right side the FEE.

NA and neutralizer: For LISA Pathfinder the neutralizer has been configured as a self-standing unit in a dedicated mechanical box where two neutralizers (main and redundant) have been allocated. The so formed NA (Neutralizer Assembly) is allocated on the external wall of the satellite and not on the same panel where the FCA is located. As a consequence of this choice and taking into account of the LISA Pathfinder space environment (L1 with very low plasma density), the neutralization function is assured by means of additional bias voltage (200V max) to enhance electron emission. Consequently being allocated far from FCA that has reduced effects of propellant contamination and electric field caused by the FEEP operation. - The neutralizer equipments for LISA Pathfinder has been developed by Thales-Alenia Space, Florence, Italy (TAS-I). 97)

PCU (Power Control Unit): The requirements of the PCU are to control any thrust in the range of 0.1 to 150 µN with a resolution better than 0.1 µN. Such a type of PCU has been developed by Galileo Avionica whose architecture can be adapted with different electrical and mechanical arrangements, providing the following main features: 98)99)

• Control and management up to four independent FEEP thrusters working in hot redundancy, providing operating voltage up to 13.7 kV at very low currents (from 0.5 µA to 2 mA)

• Control and management of two neutralizers working in cold or hot redundancy

• Single point failure tolerant architecture for its use as "primary" propulsion allowing at least three FEEP thrusters and one neutralizer fully operating in case of single failure at FEEP subsystem level (fully redundancy concept also applicable for command, telemetry and Bus power interfaces).

The most important task is to control the thrust level and to this purpose the PCU embarks two dedicated HV (High Voltage) supplies for each thruster with the main characteristics provided in Table 5. These high voltage supplies are tailored according to specific thruster needs within a maximum total voltage range of 13.7 kV. Voltages, currents and telemetries are suitable to match the two different FEEP thrusters technologies: slit-shaped emitter with Cesium propellant and needle-shaped emitter with Indium propellant.

For thrust regulation, the implemented closed loop control is allocated in the PCU and follows the control principle shown in Figure 49. It is composed of one inner analogue current loop tracking firstly the beam current parameter and then an overall digital control loop with 12 bit of resolution tracking both emitter voltage and beam current.

The thrust control performances (from the electrical point of view) have been verified with both FEEP thrusters technologies (slit and needle) providing performances in line with the requirements. The qualification of the PCU for LISA Pathfinder has been complemented by a dedicated qualification and endurance test of the high voltage power boards, exploiting the HV functions, to cover with suitable safety margin the emitter voltage extreme (i.e. in excess of 15 kV and 20% of additional power level).

On LISA Pathfinder, the capability of adaptation to different thrusters technology and neutraliser solution, without jeopardising the PCU design and implementation, reveals an excellent level of flexibility, and adequate for potential use of this PCU for future missions.

DRS is a NASA-provided instrument package within the New Millennium Program developed by NASA/JPL. When first proposed, the DRS payload was known as DRS-PFCV (Disturbance Reduction System-Precision Flight Control Validation), consisting of the GRS (Gravity Reference Sensor) design of Stanford University, which closely resembled the LTP, namely in that it consisted of two inertial sensors with the associated interferometric readout, as well as the drag-free control laws and µN colloidal thrusters (organic ionic liquid) - although the technologies employed were different from the LTP implementation. 100)101)102)103)104)105)106)

Due to budgetary problems, the descoped DRS (Oct. 2005) of the former ST7 (Space Technology 7) mission now consists of the micro-Newton (µN) colloidal thrusters, DFACS (Drag-Free and Attitude Control System), and a microprocessor. The DRS will now use the LTP inertial sensors as its drag-free sensors (test masses position and attitude) to control the spacecraft attitude with independent drag-free software and will use the colloidal thrusters as actuators.

Note: There are, in fact, two separate "DFACS packages" integrated on board the LISA Pathfinder spacecraft. LTP will utilize its own DFACS algorithm during its allocated operations period, which occurs before the DRS operations period. - During DRS operations, NASA will use its own IAU (Integrated Avionics Unit) package which includes DCS (Dynamics Control Software).

The primary goal of the DRS instrument package is to maintain the position of the spacecraft with respect to the proof mass of LTP to within 10 nm Hz-1/2 over the frequency range of 1-30 mHz. The DRS will control the spacecraft position with respect to one test mass while minimizing disturbances on the second test mass.

The conceptual functionality of the DRS system is shown in Figure 51. The two cubical test masses TM1 and TM2 are enclosed within housings rigidly attached to the body of the spacecraft. Electrodes on the inner faces of the housings are used to measure the position and orientation of the test masses with respect to the housings using a capacitive sensing mechanism. A laser interferometer is being used to measure the distance changes between the two test masses to infer the residual acceleration noise. Colloidal microthrusters are being used to counteract the external forces, which are primarily due to solar radiation pressure acting on the spacecraft solar panel. The thrust level is continually being adjusted to keep the spacecraft centered about the test masses.

Disturbances: The largest disturbances to the inertial trajectory of a spacecraft (radiation pressure, residual gas drag, and particulate impacts) are cancelled by the basic concept of a drag-reduction system. The final performance of the system will be limited by a number of smaller disturbances. These disturbances fall into three categories:

1) Variations in the gravitational potential at the test-mass location

3) Variations of the electromagnetic fields at the test-mass location.

The main gravitational fluctuations are due to the thermal distortion of the spacecraft and to the relative displacement of the test mass with respect to the spacecraft. Reducing the gravity gradient and displacement of the test mass minimizes the gravity noise caused by spacecraft displacement.

Parameter

Value

Thrust range

5 to 30 µN

Thrust precision

< 0.1 µN

Thrust noise

< 0.1 µN Hz-1/2 (5 Hz control loop)

Thrust command rate

10 Hz (< 0.1 s latency)

Thrust respond time

< 100 s from maximum to minimum

Specific impulse (30 µN point)

> 150 s

Specific impulse (6 µN point)

> 275 s

Operational lifetime

> 2,200 hours

Plume half angle

< 35% (95% beam current)

Table 6: Summary of the DRS microthrust propulsion system requirements

CMNTA (Colloid Micro-Newton Thruster Assembly), designed and developed for NASA by Busek Co. Inc. of Natick, MA. The objective is to smoothly and continuously counter all external disturbances with control authority over all six degrees of freedom (DoF) of the spacecraft motion. DRS provides a thrust level range of 5-30 µN with a resolution of 0.1 µN.
Note: Colloid thruster types are part of the EIP (Electrostatic Ion Propulsion) family. The EIP concept uses a high voltage electrostatic field to accelerate positively charged particles (or ions) to large exhaust velocities (acceleration is created by the force on charged particles in the electric field).

The CMNTA thrusters use a colloidal fluid propellant. The fluid is fed through a needle by a pressurizing system. At the tip of the needle, a high electrical field is applied, which causes droplets to form and to be ejected from the tip of the needle. The droplets of the "electrospray" are spontaneously charged and accelerated by the electric field. A typical single-emitter-needle thruster produces a maximum thrust of 3 µN. Each thruster employs an array nine needles while four thrusters are mounted on one "cluster" assembly. Two clusters of thrusters are being used for DRS. Each cluster consists of the thrusters, one carbon nanotube emitter, the propellant feed system, and the PPU (Power Processing Unit). The PPU contains all the DC-DC converters to power the system and the autonomous controls for the carbon nanotube field emission neutralizer. The full thruster cluster is shown in Figure 52.

• Thruster head: The thruster head is comprised of a manifold that feeds nine emitters and the electrodes that extract and accelerate the propellant.

• Propellent feed system: Propellant is stored in a stainless steel bellows compressed by four constant force springs set to supply the microvalve with propellant at approximately 1 atmosphere of pressure. A µValve is piezo-actuated using ~1 mW of power to control the propellant flow rate to better than 1 nA equivalent resolution. This level of precision corresponds to ≤ 0.01 N of thrust, with a response time over its full range of less than 0.5 s.

• Cathode neutralizer: The cathode neutralizer is made from a carbon nanotube (CNT) base with an extractor electrode. The cathode is capable of producing 10 µA to 1 mA using extraction voltages of 250-770 V.

• Thruster electronics: The thruster electronics consists of 4 power processing units (PPUs) and one digital control and interface unit (DCIU) for each cluster. A PPU includes the high-voltage DC-DC converters.

IAU (Integrated Avionics Unit), designed and built for NASA by Broad Reach Engineering Inc., Tempe, AZ. The flight software resides in IAU which serves as the interface among the drag-free sensors, the thrusters, and the host spacecraft. [Note: In ESA terminology, the IAU system is referred to as DFACS (Drag-Free and Attitude Control System) + microprocessor].

The IAU (Figure 53) contains a 30 MHz cPCI (Compact Peripheral Component Interface) backplane, a Rad-750 processor (main processing board), and specialized cards [CAPI (Command and Payload Interface), SMACI (State Monitor and Attitude Control Interface)] to support communications with the spacecraft and the thrusters, as well as housekeeping sensor monitoring (temperature and currents). The IAU relays science data to the spacecraft, which is responsible for downlinking data to the ground.

The DCS (Dynamics Control Software) is also part of the flight software. The flight software executes the DCS at 10 Hz. The spacecraft interface provides position and attitude measurements from the drag-free sensors, as well as the attitude and rates of the spacecraft. The DRS sends requested test mass forces and torques to the drag-free sensors. This supplements the force and torque commands sent to the colloid thrusters to act on the spacecraft.

The DCS determines the thruster commands to control the spacecraft position and attitude based on the measurements of the position of each test mass relative to its housing. The variation in thrust commanded by DCS must be within the response capability of the thrusters. The electrostatic forces and torques for the test masses are a function of the test-mass housings. The spacecraft control requirement is to keep the spacecraft centered about the two test masses within < 10 nm Hz-1/2 over the frequency range of 1 to 30 mHz.

Several disturbance models are included in the design of the controls: solar radiation pressure variation; capacitive sensing noise (modeled as a colored power spectrum); thruster and star tracker noise (modeled as white); and acceleration noise on the test mass, including magnetic and Lorentz forces, thermal variations (self gravity), and cosmic ray impacts.

The LISA Pathfinder ground segment comprises two operational centers, both provided by ESA:

• The MOC (Mission Operations Center) at ESA/ESOC. MOC is responsible for LEOP, the transfer phase, and all operations during the routine phase and is in contact with the spacecraft for eight hours per day through the ground station(s).

• STOC (Science and Technology Operations Center) is the point of interface to the scientific community, and is responsible for the payload scheduling (both long and short-term), quick-look data analysis, data processing and archiving.

The STOC will also take a leading role in the analysis of the mission data. Development of the STOC is run from the ESAC (European Space Astronomy Center) in Villafranca, Spain, however, during the science operations of the LTP, the STOC will be re-located to ESOC. This is to enable the required close contact between the science operations planing and the mission operations (Ref. 5). 109)

• LTPP (Long-Term Payload Planning): This activity is concerned with the high-level planning of the 90 days of the mission operations. In particular, in defining the experiments to be performed, and the creation of a strawman operational plan. The results of the LTPP are contained in the EMP (Experiment Master Plan)

• MTPP (Medium-Term Payload Planning): This activity concerns the validation of the POR (Payload Operational Requests). A POR is a time-tagged list of telecommand sequences to be executed autonomously. One POR is required for each day of operations.

• STPP (Short-Term Payload Planning): This activity deals with the delivery of validated PORs to the MOC for generation of the Mission Timeline.

• DI (Data Ingestion): The main purpose of the STOC Data Ingestion System is to retrieve telemetry data during each of the ground passes and to make it available to the STOC Quick-Look and DA (Data Analysis) and to the archive system for future usage by LTP and DRS.

• QL (Quick Look): The aim of the QL subsystem is to monitor the LTP operations taking place and to provide an alert in case something is not as expected. The QL uses a subsystem of full science telemetry which is directed to a specific packet-store board, and is telemetered with high priority at the start of the ground station pass. Following the QL activities, the STOC may:

- Issue a warning for a deeper investigation as part of the DA.

- Request to MOC a change of TC parameter to be applied to the next run if an immediate action is needed.

- Request to MOC to immediately command the LTP into Standby mode.

• DA (Data Analysis): The DA is a joint effort of the STOC, LTP and DRS teams and will use the telemetry and auxiliary files available in the STOC archive.

• SA (Science Archive): The SA will make all the data accumulated by LPF and a subset of the data analysis products available to the wider scientific community. The LTP team have priority rights to the data for the first three months, after which the archive goes live on the public domain.

Figure 57: Overview of the science operations ground segment showing the data flow between the various subsystems (image credit: ESA)

The information compiled and edited in this article was provided byHerbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (herb.kramer@gmx.net).